Applying protective metal coatings on refractory metals



lz-Nickel Phosphide IZ-Nickel Phosphide ls-Electroploted Chromium INVENTOR Herbert E. Ricks.

ATTORN l0 Molybdenum lG-Electroplored Nickel lo-Molybdenum H. E. RICKS APPLYING PROTECTIVE METAL COATINGS ON REFRACTORY METALS Filed July 26, 1952 Fig.l.

APPLYING PROTECTIVE METAL COATINGS ON REFRACTORY METALS Herbert E. Ricks, Pittsburgh, Pa, .assignor torwesting house Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania v Application July 26, 1952, Serial No. 301,016 r 6 Claims. or. 204-31 This invention relates to members of molybdenum metal and other similarly readily oxidized refractory metals with imperforate protective.metal coatings applied to the surfaces of the members. V

The refractory metalsmolybdenum, titanium and zirconium, are readily oxidized when exposed to an oxygenbearing atmosphere at elevated temperatures. Thus, while molybdenum metal melts at 2625 C. in anonoxidizing atmosphere, if the molybdenum metal is heated to 500 C. and higher in contact with air,oxygen or other oxidizing gas, it forms a readily volatile oxide suchthat when in contact with air at 700 C., the molybdenum metal gives off dense dark clouds of smoke consisting of molybdenum oxides and in a short while it is consumed.

It has been proposed to coat the; surface of molybdenum metal with a protective layer of chromium or other oxygen-resistant metal which would prevent access of oxygen or other oxidizing vapor tothe molybdenum and in this way enable the use of molybdenum metal at temperatures above 500 C. It is absolutely necessary that such applied protective layers be imperforate in order to be of any value. However, it has been impossible hitherto to provide an imperforate coating that would so exclude oxygen. For example, electrodeposits of chromium have been plated on members of molybdenum and such coatings appeared to be sound and without pores. However, on heating such chromium plated members to 600 C. to 700 C., microscopic cracks and previously unnoted pinholes admitted oxygen to the molybdenum metal. so that a stream of molybdenum oxide smoke issued from each of these openings and in only a short time substantial volumes ofthe molybdenum metal adjacent to each such opening had disappeared because it had oxidized and the resulting volatile oxide had escaped.

ings of electroplated metal on molybdenum, all without any consistent success. Further, in many cases, the electroplated metal did not adhere properly to the molybdenum and on heating blisters were formed with resulting failure.

For making members for high temperature applications as in gas turbines, jet engines and other highly heated applications demanding great strength, molybdenum metal members would be satisfactory if they were protected by a well-bonded adherent coating of a metal resistant to oxygen at the elevated temperaturesv encountered in such service- The object of the present invention is to provide members comprising a body of molybdenum, titanium or zirconium, with the entire surface of the body coated with a highly adherent, imperforate deposit of nickel phosphide, the applied deposit being partially diffused into the surface of the body.

A further object of the invention is to provide for applying to molybdenum, titanium, and zirconium metal bodies an imperforate surface coating of nickel phosphide 2 and heat treatingthe body tire surface of the bodyJ Other objects of the invention will in part be obvious and will in part appear hereinafter. For a" better under? standing of the nature and objects ofthe invention, 're ference should be had to the following detailed description and drawing, in which:

Figure 1 is a cross section through a membercompris ing a body of molybdenum with a bonded coatingl'of nickelphosphide over the surface thereoftand Fig. 2 is a cross section through a molybdenum member having applied to its entire outersurface a nickel phosphide layer and a superimposed electrodeposit of at'least one of the group mium.

I have discovered a method for producing on members of molybdenum, titanium or zirconium metal an imperforate, highly protective surface coating of the order of 0.001 inch thickness, which coating is so well bonded and adherent to the metal that it will withstand eIeVated'temperatures and will protect the metal from oxidizing at-' mospheres at such elevated temperatures for prolonged periods of time. Briefly, the method comprises plating on cleaned molybdenum metal a coating of nickel phosphide, cobalt phosphide or both over the entire surface of the member to a thickness of the order of 0.001 inch and then heat treating the member with the applied nickel melt the nickel phosphide coating.

sulting imperforate coating is so well bonded tothe re:-

fractory metal that it will not crack open or part there from under extreme conditions of service. .Eurthermore, the heat-treated nickel phosphide coatingfmay be readily overplated withone or more ele ':trodep'osits v of any of the metals of the group consisting of nickel,

cobalt and chromium.

The invention. will be detailed hereinafter by reference to molybdenummetal but it will be understood similar procedures may be applied to titanium and zirconium metal. The invention may be applied not only to pure molybdenum, titanium and zirconium metal, respectively, but to alloys in which molybdenum, titanium and zirconium constitute at least 50% by weight. Thus, the invention may-be applied to members compris ing molybdenum and 5% tungsten. Titanium may be modified with small amounts of chromium. 'Allo'ys'bf titanium containing a few percent of'chromiumarnifiup to 8% molybdenum, for example, may be treated in accordance with the present invention; It will be under? stood that zirconium ordinarily may containa substantial; amount of hafnium. Alloys containing 'mixturesiofa'ny two or all three of the metals molybdenum, titanium and zirconium with small amounts of various. additionalel e me'nts may be treated in accordancewith the presenting Example :1

Members of molybdenum metal are.cleanedby tirst mechanically brushing, grinding and otherwise treating the surface to remove any impurities and blemishes thereon. The clean molybdenum, member "is'then mersed in the following electrolyte:

2,739,107. Patented Mar. 20, 1956 with the applied coatingto' produce a well-bonded, imperforate coating over the enconsisting of nickel, cobalt and chr'of Ounces per gallon Nickel sulphate 3 Nickel chloride 7 Boric acid 4 Nickel I phosphite 5 molybdenum member is-then placed in a furnace provided with a hydrogen atmosphere and is heatedslowlyto a temperature of 550 C. over a period of time of four hours. The furnace temperature is then raised in 30 minutes to 800C. The molybdenum member is maintained in the cooling chamberportion of the furnace where it does not reach a temperature much above 800 C., and thereafter the furnace temperature is increased to 1000" C. The plated molybdenum metal is then moved from the cooling chamber into the portion of the furnace. at 1000 C. and kept there for minutes. 'Ihe molybdenum metal'is then withdrawn and cooled to room temperature.

Members prepared as in Example I may be subjected to creep testing at 1800 F. at a loading of 20,000 p. s. i. At the end of many hours, the coating will be found to be completely impervious and will have perfectly protected the molybdenum metal from oxidation. The applied nickel phosphide coating will be oxidized only slightly. The nickel phosphide coating will melt when heat treated and any excess above approximately 0.001 inch thickness will run off the member.- After complete heat treatment, it has been found that the nickel phosphide comprises from 90 to 99% nickel with the balance being phosphorus. The average thickness of the heat-treated nickel phosphide coatings is approximately 0.0007 inch when it was originally plated to at least this thickness. If the plated thickness is less than 0.0007 inch, then the heat-treated nickel phosphide coating is the same thickness. The meltingof the nickel phosphide deposit is necessary to produce an imperforate coating consistently. Referring to Fig. 1 of the drawing, there is illustrated a cross section of a member produced in accordance with the, process of this example. The body 10 of molybdenum metal has applied to all of its surfaces an exterior coating 12 of the heat-treated nickel phosphide deposit of a thickness of the order of 0.001 inch. The junction 14 between the nickel-phosphide deposit 12 and the body 10 shows a slight difiusionand is characterized by no blisters, breaks or other perforations or faults.

Theabove aqueous electroplating electrolyte may be varied to provide, from to ounces per gallon of nickel sulphate, from 6 to 9 ounces per gallon of nickel chloride hexahydrate, from 3 to 5 ounces per gallon of boric acid and from 4 to 6 ounces per gallon of the nickel phosphite. Good results have been secured in plating at current densities from 5 to 50 a. s. f. The pH of the electrolytes may vary from about0.5 to 2 while the temperature of the electrolyte may vary from about 65 C. to 85 C:

The heat treatment of the applied nickel phosphide should be carried out in a non-oxidizing atmosphere such members in a vacuum, thereby'elirrunating any chance of contamination or causing,undesirable results because oi slight impurities in, the Jneatatreating gas. atmosphere SUIIQUJJdiDg;I-tl8 membtensr,

In one. test, nickel phosghide was laced'in. an evacur ated furnace chamber andorr raising the temperature above 1200 C., the nickel phosphide vapors coated surfaces of. members. in. the. furnace. Titanium and zir: conium members may be readily coated with nickel phosphide in such a vacuum furnace, the nickel phosphide being heated while the titanium or zirconium members are much cooler.

In order to provide better. surface protection for resisting mechanical forces and wear andtear, the heat-treated nickel phosphide depositis over-plated with one or more layers of at least one metal consisting of the group nickel. cobalt and chromium. Thus, a-deposit of 0.002 inch of nickel or cobalt or alloys thereof may be plated over the nickel phosphide coating and thena substantial layer of chromium may besuperirnposed thereon; The following examples illustrate this feature.

Example If A rod of molybdenum one-fourth inch in diameter was plated in the electrolyte of Exampleil to produce thereon a nickel phosphide layer of a thickness of 0.001 inch. The platedrodwas then placed in a furnace provided with a cracked ammonia atmosphere and heated in accordance with the schedule set,forth under Example I. 'Ihe heat-treated rodwas then placed in a nickel electrolyte having 40 ounces per-ga1lon,of nickel sulphate heptahydrate, 6 ounces per gallon nickel. chloride hexahyclrate,

and 5 ounces per gallonof boric acid'and plated at a current density of 40 amperesper square foot until a layer of nickel of athickness of 0:002 inch was deposited thereon. The rod was then'placedin a creep tester at 1800 F. under a load of-2 0,000 p; s. i. After passing 40 hours of this-test, the rod'was examined'and was found to be in perfect condition and completely protected by the. surface coating. Photomicrographs of cross sections through the rod showed aperfect bondbetweeri the nickel phosphide coating and the molybdenum metal. A slight oxidation was observed at the surface of the nickel coating dueto its exposuretothe air at 1800" F. for the test.

Example III A rod was, prepared by plating thereon 0.001 inch of nickel phosphide, heat treated as in. Example I, and 0.002 inch thickness of nickel was plated thereon as in Example II', and the rod was further platedin a chromic acid plating electrolyte comprising 53,0unces per gallon of chromatic acid and 0:53 ounce per gallon of sulphate, to deposit a 0.0015 inch thickness of chromium over the nickel layer. The rod was then placedin a creep tester whereit' was loaded at 20,000 p; s. i. at a temperature of 1800 F. After hours under the test without failure, the rod was examined'and found'to be free from any oxidation of the underlying molybdenum metal. From all indications, the rod could, have withstood this load at these temperatures for a much longer period of time.

Referring to Fig. 2 of the drawing, there is illustrated a cross section througha member produced in accordance with this Example III. A body IOofmOlybdenum has plated thereon the nickel phosphide layer 12, then-superposed thereon is a layercomprising electroplated nickel l6 and finally an exterior layer 13 comprising the electrodeposited chromium.

The nickelphosphide layerrnay'be plated to a thickness of from 0.0001- to OLOOZ inch; the deposits thinner than about 0.0007 inch preferably being protected by a superimposed" coating ofnickel. cobalt or chromium. During'the heattreatment of the applied nickel phos phide coating, the coating melts andbecomes imperforate, and partly diffuses and forms a highly adherent bond" to the molybdenum metal surface. Themelting point depends on theamount-ofphosphorus in the metal phosphide. Also, duriugheat treamaent. a substantial part of the phopborus leaves the coating, mostly by volatilization.

Cobalt sulphate, cobalt chloride and cobalt phosphite may be-substituted'in whole or irrpart for the nickel sulphate, nickel chloride, and nickel phosphite, respectively, in Examples I to III, thereby producing a composite cobalt-nickel phosphide deposit or an entirely cobalt phosphide deposit. These cobalt-containing phosphide deposits are otherwise treated identically ,as set forth herein.

While the electroplating process disclosed herein has given excellent results, it is feasible to produce a similar nickel phosphide coating by an immersion process wherein the molybdenum metal is immersed in an electrolyte of the type described in the November 1947 issue of the Journal of Research of the National Bureau of Standards in an article entitled Deposition of nickel and cobalt by chemical reduction. Briefly, one such immersion solution comprises 30 grams of nickel chloride hexahydrate, 50 grams of ammonium chloride and 100 grams of sodium citrate per liter of solution, the balance being water. There is then added sufiicient ammonium hydroxide to render the solution alkaline to a pH of from 8 to 10. When immersion plating is to be accomplished therein, the solution is heated to a temperature of from 190 to 200 F. and then 10 grams of sodium hypophosphite is added to each liter of solution. For every hour'that plating is carried out in the solution, sodium hypophosphite is added to the bath at the rate of grams per liter. This plating bath will build up a coating at the rate of 0.0003 inch per hour. Therefore,

in an immersion period of from two tothree hours, a-

sufi'icient thickness of nickel phosphide will be produced on the molybdenum metal. The resulting deposit is heat treated similarly to the nickel phosphide deposit described in the previous examples. cobalt salts may be substituted in whole or in part for the nickel salts in this immersion bath.

Since certain obvious changes may be made in the above procedures andditfer'ent embodiments of the invention may be made without departing from the scope thereof, it is intended that all matter contained in the above description and drawing shall be interpreted as illustrative and not in a limiting sense.

I claim as my invention:

1. A member comprising a body of molybdenum metal and an imperforate coating of a thickness of the order of 0.001 inch applied to the entire surface of the body of molybdenum metal, the coating comprising essentially over 85% by weight of at least one metal selected from the group consisting of cobalt and nickel, and the balance being phosphorus, the applied coating being melted at from 600 C. to 1100 C. and partly diffused into the surface of the molybdenum.

2. A member comprising a body composed of at least 50% by weight of molybdenum metal and an imperforate coating of a thickness of substantially more than 0.001 inch comprising at least two layers applied to the entire surface of the body of molybdenum metal, the innermost layer of the applied coating being approximately 0.001 inch in thickness and comprising essentially over 85% by weight of at least one metal selected from the group consisting of nickel and cobalt, and the balance being phosphorus, the applied innermost layer being melted at from 600 C. to 1100 C. and partially difiused into the molybdenum, andsuperimposed on the It will be appreciated that innermost layer an adherent electrodeposit 'of' at least one metal of the group consisting of nickel, cobalt and chromium.

3. A member comprising a body of a metal selected from at least one of the group consisting of molybdenum,

titanium, zirconium and alloys comprising at least 50% by weight thereof, an imperforate metal phosphide coating over the entire surface of the body of the metal, the coating comprising essentially at least 90% by weight of atleast one metal selected from the group consisting of nickel and cobalt and the balance being phosphorus, the coating being of a thickness of the order of 0.001 inch and having been melted by heat treatment at a temperature of from 600 C. to 1100 C. after being consisting of nickel and cobalt and the balance being phosphorus, the coating being of a thickness of the order of 0.001 inch and having been melted by heat treatment at a temperature of from 600 C. to 1100. C. after being applied to the body, and an electrodepositsuperimposed on the metal phosphide coating, the electrodeposit, comprising at least one metal selectedfrom the group consisting of nickel, cobalt and chromium.

5. .In'the process of producing-imperforate protectivecoatings on a body of a metal selected from at least one of the group c'onsistingof molybdenum, titanium and zirconium and alloys comprising at least 50% by weight thereof, the steps comprising electroplating on the entire surface of the body of the metal a thicknessiof the order of 0.001 inch .of a metal phosphide composed of at least metal and the balance being phosphorus'from an electrolyte containing the chloride and from about 4 to 6 1 ounces per gallon of phosphite of at least one metal from the group consisting ofnickel and cobalt, and heat treating the body of the electroplated metal phosphide coating at a temperature of from 600 C. to 1100 C. in a non-oxidizing, inert atmosphere to melt the electroplated coating so as to render it imperforate and to diffuse it into the body of the metal, the melted coating composed of from to 99% by weight of the metal and the balance being phosphorus.

6. In the process of producing imperforate protective coatings on a body of a metal selected from at least one of the group consisting of molybdenum, titanium and zirconium and alloys comprising at least 50% by weight thereof, the steps comprising electroplating on the entire surface-of the body of themetal a thickness of the order of 0.001 inch of a metal phosphide from an electrolyte containing the chloride and from about 4 to 6 ounces per gallon of a phosphite of at least one metal from the group consisting of nickel and cobalt, heat treating the applied metal phosphide coating at a temperature of from 600 C. to 1100 C. in a non-oxidizing" atmosphere to melt it, and thereafter plating at least one metal from the group consisting of nickel, cobalt and chromium over the heat-treated metal phosphide coating.

References Cited in the file of this patent UNITED STATES PATENTS Brenner et al June 23, 1953 

5. IN THE PROCESS OF PRODUCING IMPERFORATE PROTECTIVE COATINGS ON A BODY OF A METAL SELECTED FROM AT LEAST ONE OF THE GROUP CONSISTING OF MOLYBDENUM, TITANIUM AND ZIRCONIUM AND ALLOYS COMPRISING AT LEAST 50% BY WEIGHT THEREOF, THE STEPS COMPRISING ELECTROPLATING ON THE ENTIRE SURFACE OF THE BODY OF THE METAL A THICKNESS OF THE ORDER OF 0.001 INCH OF A METAL PHOSPHIDE COMPOSED OF AT LEAST 85% METAL AND THE BALANCE BEING PHOSPHORUS FROM AN ELECTROLYTE CONTAINING THE CHLORIDE AND FROM ABOUT 4 TO 6 OUNCES PER GALLON OF PHOSPHITE OF AT LEAST ONE METAL FROM THE GROUP CONSISTING OF NICKEL AND COBALT, AND HEAT TREAT- 